In recent years, with the object of achieving weight reduction of automobiles, which leads to improvements in the fuel efficiency thereof, efforts at reducing the weight of steel sheets used in automobiles by using steel sheets having an increased strength are progressing. However, if the strength of steel sheets becomes high, when automotive parts are manufactured by press forming, problems occur such as the occurrence of galling or breakage of steel members, or the shape of formed articles becoming unstable due to the springback phenomenon.
One technique for manufacturing high strength parts is a method in which instead of carrying out press forming of a high strength steel sheet, press forming is carried out in a low strength state, and quench hardening is carried out after press forming in order to increase the strength of the steel sheet. In this method, the steel composition of the steel sheet is selected so that hardening can be achieved.
Another technique is hot press forming in which a steel sheet is heated and then subjected to press forming. In hot press forming, the steel sheet decreases in strength due to heating, so press forming can be carried out without the above-described problems, even with a steel sheet having a relatively high strength at room temperature.
It is possible to combine hot press forming with hardening so as to obtain a part with even higher strength. Thus, after a steel sheet having a steel composition capable of hardening is heated to a temperature sufficient for hardening, it is subjected to press forming at that temperature and then to quenching for hardening either inside the press die or after the press forming step is finished. As a result, compared to the case in which only hardening or hot press forming is employed, it is possible to manufacture a formed part of even higher strength. Therefore, in hot press forming, hardening is usually performed. If hardening is carried out inside a press die, forming and hardening can be simultaneously achieved without performing heat treatment for the purpose of hardening after press forming. For example, see JP-A 2002-102980.
Since hot press forming is a method in which working is applied to a heated steel sheet, surface oxidation of the steel sheet is unavoidable. Even if a steel sheet is heated in a nonoxidizing atmosphere, while it is being removed from a heating furnace and subjected to pressing, it contacts the air, and iron oxides are formed on the surface of the steel sheet. Moreover, heating in such a nonoxidizing atmosphere increases costs.
Iron oxides which are formed on the surface of a steel sheet may drop off and adhere to a die during pressing, thereby interfering with productivity or making the appearance of a press-formed article poor due to a film made of such iron oxides (referred to below simply as an iron oxide film) remaining on the article. Moreover, if such an iron oxide film remains on a formed article, the iron oxide film has poor adhesion to the surface of the formed article, so if chemical conversion treatment and coating are performed in a subsequent step without removing the iron oxide film, problems occur with respect to coating adhesion, and as a result, corrosion resistance after coating decreases.
As described in claim 6 of JP-A 2003-2058, prior to coating a hot press-formed article, the surface of the formed article is normally cleaned by sandblasting or shot blasting treatment to remove an iron oxide film from the surface. However, such blasting treatment is troublesome, and it greatly decreases the productivity of hot press forming. In addition, it may produce strains in the formed article.
In order to suppress the formation of iron oxides due to oxidation of the surface of a steel sheet during hot press forming and to impart corrosion resistance after forming, it is proposed to apply hot press forming to a steel sheet with a zinc-based plating in JP-A 2001-353548 and JP-A 2003-73774 or to a steel sheet with an aluminum-based plating in JP-A 2000-38640.
However, when an aluminum-plated steel material is hot pressed, when the steel material is heated, mutual diffusion occurs between the plating layer and the steel base material, and Fe—Al or Fe—Al—Si intermetallic compounds are formed in the plating interface. Since these intermetallic compounds are harder than the plating layer at the temperature of hot press forming, they impart damage to the die used for hot press forming at the time of continuous press forming, leading to damage to the appearance of the formed articles.
In addition, in hot press forming of an aluminum-plated steel material, an oxide film of aluminum is formed on the plating surface at the time of heating. The oxide film of aluminum causes problems with respect to coating adhesion when coated, though the problems are not as severe as those caused by an oxide film of iron, and therefore the high level of coating adhesion which is demanded of materials for outer plates and suspensions of automobiles cannot necessarily be achieved. In addition, the oxide film makes it difficult to form a good film by chemical conversion treatment which is widely used as pretreatment before paint coating.
Likewise, in hot press forming of a zinc-based plated steel material, iron-zinc (Fe—Zn) intermetallic compounds may form by mutual diffusion between the plating layer and the steel base metal. These intermetallic compounds are also hard, so they impart damage to a hot press forming die. Alternatively, if the heating is inadequate so that a pure zinc phase remains, since the melting point of zinc is lower than the hot press forming temperature, there are cases in which molten zinc is scattered during hot press forming and contaminates the die.
In the hot press forming of a zinc-based plated steel sheet proposed in JP-A 2003-73774, a zinc oxide layer is previously formed on the surface of the zinc-based plating layer by, for example, heating such as one employed in galvannealing finish. The zinc oxide layer on the surface of the plated steel sheet functions as a barrier layer which prevents vaporization of the zinc-based plating layer during hot press forming and during heating prior thereto. However, that publication does not mention anything about the presence of a zinc oxide layer on the surface of a press-formed article formed by hot press forming or the effect thereof. It contains a description that approximately 0.01-5.0 μm is sufficient for the thickness of the oxide film (namely, a zinc oxide layer) which functions as a barrier layer, but this is the thickness of a barrier layer for preventing vaporization of zinc during hot press forming. Therefore, this is the thickness of a zinc oxide layer on the surface of a zinc-based plated steel sheet prior to heating to a hot press forming temperature, and it is not the thickness of a zinc oxide layer on the surface of a formed article obtained by hot press forming.
A hot press-formed article made of a zinc-based plated steel sheet is frequently coated after forming and then used as a part. Such a coated part typically has improved corrosion resistance compared to a part formed by coating a hot press-formed article made from a bare steel sheet.
In the examples of JP-A 2003-73774, the post-coating corrosion resistance of a hot press-formed article made from a zinc-based plated steel sheet is tested. Specifically, the case in which the width of swelling is less than 4 mm in a salt spray test performed for 480 hours is evaluated as “good in post-coating corrosion resistance”.
However, the level of post-coating corrosion resistance which is demanded is continually increasing, and depending upon the type of part, the above-described standard for evaluation is inadequate. Examples of parts for which a particularly high post-coating corrosion resistance is demanded are outer panels and suspensions of automobiles.
Accordingly, in the technical field of hot press forming of galvanized and similar zinc-based plated steel sheets, there is a need for a formed article which has good strength as well as good coating adhesion and improved post-coating corrosion resistance when coated and which can be stably manufactured with high productivity. It is also desired that it have a good appearance and that it have good spot weldability, since hot press-formed articles are often assembled or installed by spot welding.